The disclosure of Japanese Patent Application No. 2000-353446 filed on Nov. 20, 2000 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
The invention generally relates to an estimation of a maximum friction coefficient between a tire and a road. More particularly, the invention relates to an apparatus and method for estimating the maximum friction coefficient between a tire of each wheel and the road, whether each particular wheel is a driving wheel or a driven wheel.
2. Description of Related Art
An apparatus for estimating the maximum friction coefficient between a tire and a road in vehicles such as automobiles is disclosed in, e.g., Japanese Laid-Open Publication No. HEI 3-295445. The apparatus for estimating the maximum friction coefficient as described in the aforementioned publication calculates a driving torque and a vertical load of a wheel when the wheel is placed into a predetermined acceleration slip state, and calculates the maximum friction coefficient between the road and the tire based on the calculated driving torque and vertical load. This type of maximum friction coefficient estimating apparatus is known as related art.
Such a maximum friction coefficient estimating apparatus estimates the maximum friction coefficient based on the driving torque and the vertical load of the tire at the time the driving wheel is placed into a predetermined acceleration slip state. Therefore, this apparatus is capable of accurately estimating the maximum friction coefficient between the tire and the road as compared to, for example, an apparatus for estimating a friction coefficient based on the square root of the sum of squares of the longitudinal acceleration and the lateral acceleration of the vehicle.
However, such a conventional maximum friction coefficient estimating apparatus can estimate the maximum friction coefficient only at the moment the driving wheel is placed into a predetermined acceleration slip state. Moreover, in order for this estimating apparatus to estimate the maximum friction coefficient, the wheel must be placed into the predetermined acceleration slip state. Therefore, this estimating apparatus cannot estimate the maximum friction coefficient between the tire of the driven wheel and the road.
The invention is made in view of the foregoing problems in the conventional maximum friction coefficient estimating apparatus that is configured to estimate the maximum friction coefficient based on the driving torque and the support load of the tire when drive slip occurs.
As the slip ratio of the tire increases, the road friction coefficient gets closer to the maximum friction coefficient, and the ratio of variation in reaction force of the road to variation in slip ratio gradually gets closer to zero. Moreover, provided that the reaction force of the road to the tire and the vertical load of the tire are obtained, the road friction coefficient (adhesion coefficient) can be obtained by dividing the reaction force of the road by the support load. In view of these points, the invention is capable of estimating the maximum road friction coefficient regardless of whether the tire is in a predetermined acceleration slip state and whether the wheel is a driving wheel.
A controller for estimating a maximum friction coefficient according to the invention includes: a first section that calculates a reaction force of a road to a tire of a wheel based on a model of the tire; a second section that calculates a vertical load of the tire of the wheel; a third section that calculates a ratio of the reaction force of the road to the vertical load as a first ratio; a fourth section that calculates as a second ratio a ratio of variation in the reaction force of the road to variation in a slip ratio of the tire, the slip ratio being calculated based on the tire model; and a fifth section that calculates a maximum road friction coefficient based on a product of a predetermined coefficient and the second ratio, and the first ratio.
Thus, the maximum road friction coefficient is calculated regardless of whether the wheel is in a predetermined acceleration slip state. Moreover, the maximum road friction coefficient is calculated either for the driving wheel or the driven wheel.